Zero pulsation pump

ABSTRACT

A positive displacement pump includes at least two pumping chambers and associated plungers. Each plunger is driven by an associated variable speed motor, such as a stepper motor, in a reciprocating motion. The stepper motor varies speed during each stroke of the plunger. A controller controls speed and direction of each stepper motor. Each stepper motor is coupled to a leadscrew having an associated guide rod mounted on the leadscrew to move along the leadscrew as the leadscrew rotates and actuate an associated plunger. The controller varies the speed, displacement and duration of the stepper motors&#39; steps to maintain a constant outflow without pulses.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention is directed to a pump producing a constant outflow and avoiding pulses in the output flow.

Description of the Prior Art

Conventional positive displacement pumps use pistons or plungers to displace fluid. However, the reciprocating motion produces pulses or surges in the output flow. It can be appreciated that single cylinder pumps generally produce the greatest pulsation as the one cylinder alternates from suction to discharge. To overcome this problem, more cylinders may be added that utilize an overlapping output that is timed to greatly decrease the pulsations. However, even with multiple cylinders, such as Quintuplex pumps, there are still small pulses.

Attempts have been made to reduce the pulsations by modifying the stroke of the plungers to reduce the pulsation. Such a pump is shown for example in U.S. Pat. No. 5,145,339 to Lehrke, which is directed to a two cylinder pump using cams to produce a pulseless output. Although the pump design in the Lehrke patent does reduce the pulsation, Lehrke's use of cams with complex cam shapes and related mechanisms are relatively complex and expensive. The design also has a fixed stroke profile that can't be varied with pumping conditions that affect the output per stroke, such as the compressibility of fluid. A design utilizing cams does not provide flexibility to alter the stroke profile while the pump is running. For high pressure metering applications, the displacement is small compared to the system bulk modulus and a portion of the stroke is needed to build pressure in the pumping chamber before any fluid leaves the discharge valve. With such a design, the cam profile for such an application would only be effective at one pressure. Different cams with different profiles would be required to be effective at different pressures.

It can therefore be seen that a simple and inexpensive positive displacement pump is needed that substantially eliminates pulses in the output flow. Such a pump should be simple and operate with a minimum number of pumping chambers and plungers to minimize cost. Moreover, such a pump should be effective even with two pumping chambers. It should also be possible to change the stroke profile and to change the profile while the pump is running. Such a pump should also be able to adapt to different operating conditions including different pressures. The present invention addresses these problems as well as others associated with eliminating pulses from positive displacement pumps.

SUMMARY OF THE INVENTION

The present invention is directed to a pulseless positive displacement reciprocating pump, such as a piston type pump or a diaphragm pump. In place of a conventional cylinder and piston arrangement, the pump preferably has two or more guide bores with associated guide rods reciprocating back and forth within the bore. The guide rod threadably connects to a lead screw, such as a recirculating ball type lead screw. An associated variable speed motor, such as a stepper motor or a servo motor, actuates each lead screw in both directions. The lead screw includes threads that mate with complementary threads of the guide rod so that as the lead screw rotates, the guide rod is moved axially along the lead screw to extend and retract the plunger into and out of the pumping chamber. The stepper motors can be precisely rotated in miniscule discrete stepped movements and are able to precisely control rotation of the lead screw and therefore the axial position of the guide rod to closely control the position of the plunger and its movements into and out of the pumping chamber.

In operation, to start the pump, with the plunger at bottom dead center, the controller would actuate a first one of the stepper motors to rotate a respective leadscrew in a first direction, which causes the guide rod to travel axially. The controller would communicate to the first stepper motor to increase the rotation speed up to the maximum that produces the full output of the guide rod and guide bore. For most of the stroke, the stepper motor rotates at a constant speed. However, as the plunger nears top dead center, the stepper motor is slowed by the controller until the plunger stops at top dead center. The stepper motor would then start turning in the opposite direction and increasing to a maximum speed and then decreasing again near bottom dead center until coming to a stop at bottom dead center.

In a two pumping chamber pump, duration of the pressure stroke is slightly longer than the suction stroke so that the ramp up portion and ramp down portion of the two plunger strokes overlap. This overlap produces the continuous combined flow at the pump discharge. As the plunger proceeds forward from bottom dead center, there is a portion of the stroke that is compressing the fluid and expanding the pressure containing components. During this period of the stroke, no flow exits the pumping chamber while the other guide rod and plunger assembly is still producing full flow. The controller can determine the number of steps that the stepper motor must move before beginning to slow the other guide rod and associated plunger. It can be appreciated that operation of a diaphragm pump would be substantially the same as for a piston type pump, but acting through a diaphragm.

For a pump with three pumping chambers, the pump stroke cycle can be timed with the overlap similar to a shaft driven pump where the strokes have a phase difference 120 degrees apart or ⅓ of a pump cycle apart. Therefore, there is overlap timing on both the pressure and suction strokes of each plunger. The speeds during the overlaps are calculated in the same manner for both the pressure and suction strokes and the displacement of and duration are precisely controlled by the controller and the stepper motors.

These features of novelty and various other advantages that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings that form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like reference letters and numerals indicate corresponding structure throughout the several views:

Referring now to the drawings, wherein like reference letters and numerals indicate corresponding structure throughout the several views:

FIG. 1 is a side sectional view of a reciprocating pump according to the principles of the present invention;

FIG. 2 is a detail view of a conventional recirculating ball type leadscrew and nut;

FIG. 3 is a perspective view of the pump shown in FIG. 1 with a portion of the housing removed for clarity;

FIG. 4 is a top sectional view of the pump shown in FIG. 3;

FIG. 5 is a side sectional view of a diaphragm pump according to the principles of the present invention;

FIG. 6 is a graph showing the output for the pump shown in FIG. 3 operating at high pressure;

FIG. 7 is a graph showing the output for the pump shown in FIG. 3 operating at low pressure with insignificant fluid compression;

FIG. 8 is a graph showing the output for a three pumping chamber pump operating at low pressure;

FIG. 9 is a graph showing the output for a three pumping chamber pump operating at high pressure; and

FIG. 10 is a flow diagram for the method for controlling the pump shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and in particular to FIGS. 1, 3 and 4, there is shown a positive displacement reciprocating pump, generally designated (100). The pump (100) includes a pump housing (102) and a manifold (104). The pump (100) includes at least one guide bore (106) and preferably two or more guide bores (106). Each guide bore (106) includes an associated guide rod (108) reciprocating longitudinally back and forth within the guide bore. The manifold (104) includes a hydraulic chamber (110) as well as an inlet check valve (112) and outlet check valve (114). A plunger (130) connects to the guide rod (108) and moves back and forth into the hydraulic chamber (110).

The guide rod (108) threadably connects to a lead screw, such as a recirculating ball type lead screw (122). A variable speed motor, such as a servo motor or a stepper motor (120), depending on the size an application of the pump, actuates the lead screw (122) in both directions. As shown in FIG. 2, a conventional lead screw (122) includes threads (124) that mate with complementary threads of a nut (126). The nut (126) may include a threaded portion (128). The lead screw (122) and nut (126) may be configured as a ball screw with a recirculating ball nut to reduce friction. It can be appreciated that the guide rod (108) may be directly driven by a lead screw as shown in FIG. 1 or mounted to the nut (126) shown in FIG. 2. Referring again to FIG. 1, a tab (132) extends from the guide rod (108) and passes through a position sensor, such as a slot detector (134) that communicates to a microcontroller (150) when the plunger (130) is at bottom dead center. The guide rod (108) includes a second tab (140) that moves in a slot (142) to prevent free rotation of the guide rod (108). Therefore, as the lead screw (122) rotates, the guide rod (108) will move axially along the lead screw to extend and retract the plunger (130) into and out of the pumping chamber (110), which is filled with fluid. It can be appreciated that the stepper motors (120) can be precisely rotated in miniscule discrete stepped movements and are able to precisely control rotation of the associated lead screws (122) and therefore the axial position of the guide rod (108) to closely control the position of the plunger (130) and its movements into and out of the pumping chamber (110).

Referring now to FIG. 5, there is shown another embodiment of a pump system (200) according to the principles of the present invention. In the embodiment shown, the pump (200) is a diaphragm type pump. The diaphragm pump (200) preferably includes at least two guide bores (206), but more typically includes three or five guide bores. The pump (200) includes a pump housing (202) and a manifold (204). The manifold includes a pumping chamber (218), an inlet check valve (212) and outlet check valve (214). The guide bore (206) includes a reciprocating guide rod (208) that connects to a plunger (230) that pumps hydraulic fluid into the hydraulic chamber (210). The hydraulic fluid acts against a diaphragm (216) to force pumped fluid from the pumping chamber (218) to the outlet valve (214). Each of the guide bores (206) is driven by a variable speed motor, such as a servo motor or a stepper motor (220) in a manner similar to that for the embodiment shown in FIG. 1. The pump (200) also includes a lead screw (222), a plunger (230), a position sensor including a tab (232) and corresponding slot detector (234), tab (240) and slot (242) to prevent free rotation, and a bearing (244). Moreover, valves (236) control the hydraulic fluid within the housing (202) as the fluid moves between suction and power strokes. Actuation of a stepper motor (220) rotates the associated lead screw (222) and moves the guide rod (208) into and out of a receiving portion of the guide bore (206). A microcontroller (250) controls each of the stepper motors (220) to achieve a varied speed and precise displacement to achieve a substantially pulse free output flow.

In operation, to start the pump (100), with the plunger (130) at bottom dead center, the controller (150) would drive a first stepper motor (120) to rotate the leadscrew (122) in a first direction, which causes the guide rod (108) to travel axially forward and therefore to the right towards the manifold as depicted in FIG. 1. The controller (150) would communicate to the stepper motor (120) to increase the rotation speed up to the maximum that produces the full output of the guide bore (108) and plunger (130) assembly. For most of the stroke, the stepper motor (120) rotates at a constant speed. However, as the plunger (130) nears top dead center, the stepper motor (120) is slowed by the controller (150) until it stops at top dead center. The stepper motor (120) would then start turning in the opposite direction and increasing to a maximum and then decreasing again near bottom dead center until coming to a stop at bottom dead center.

In a two pumping chamber pump with two plungers, duration of the pressure stroke is slightly longer than the suction stroke so that the ramp up portion and ramp down portion of the two plunger strokes overlap. This overlap produces continuous combined flow at the pump discharge. As the first plunger (130) proceeds forward from bottom dead center, there is a portion of the stroke that is compressing the fluid and expanding the pressure containing components. During this period of the stroke, no flow exits the pumping chamber (110) of the first plunger (130) while the other plunger is still producing full flow. The controller (150) can determine the number of steps that the stepper motor (120) must move before beginning to slow the other guide rod/plunger. It can be appreciated that operation of a diaphragm pump (200) would be substantially the same as for a piston type pump (100).

The controller utilizes a formula of

Dv=P*V/K

where:

-   -   K and V are constants stored in the microcontroller for the         pump.     -   P=the output pressure and can be entered by the operator or         input from a transducer.     -   K=the combined bulk modulus of the system which includes the         fluids and components that deflect during the pressure stroke.         This can be determined experimentally for the pump being         controlled.     -   V=pumping chamber volume     -   dV=the change in volume due to compression of the fluids and         expansion of the pumping chamber.

Example using values for a small metering pump operating at 3000 psi.

-   -   P=3000 psi     -   V=0.4 cubic inches     -   K=250,000 psi     -   dP=3000*0.4/250000=0.0048 cubic inches.

dP divided by the plunger diameter will give the stroke travel to build up to system pressure from 0.

The controller (150) calculates the number of steps that the stepper motor (120) moves to advance the plunger (130) from bottom dead center before the fluid starts leaving the pumping chamber (110) and the other plunger (130) can start slowing down. Stepper motors have a fixed number of steps per revolution. A typical motor uses 200 steps per revolution. However, drivers can increase this number to a much higher number of steps per revolution using microsteps for each full step. Stepper motors may commonly have as many as 3200 steps per revolution for certain applications. As each step is a fixed amount of rotation and the lead screw (122) moves the guide rod (108) a constant linear travel distance per revolution, each step will correspond to a fixed axial displacement and therefore a fixed volume of displaced fluid. Therefore, the time for each step determines the output flow rate of the corresponding plunger and pumping chamber. With these parameters being known, the constant output per step makes an algorithm for flow rate and combining flows from multiple pumping chambers and associate guide rods and plungers can be calculated as follows:

The flow rate for one plunger is governed by the formula:

Q=Vs/T

-   -   Where:     -   Q is the volume of fluid per step Vs divided by the duration for         the step T.

The microcontroller 150 controls the stepper motor speed by varying the duration T. The controller varies the duration T continuously during the ramp pressure or combined flow periods of the stroke.

When multiple plungers are producing flow the total flow Qt is calculated as follows:

T=T1+T2 for a two plunger pump

Where:

-   -   T1 and T2 are the duration of the step for motors 1 and 2.

Therefore, the volume of fluid can be shown as:

Qt=Vs/(T1+T2)

It can therefore be appreciated that when only one guide rod/plunger is operating, T1=T, and T2=0. When a first plunger starts to slow as it reaches the end of the stroke, the second plunger starts moving at a speed determined by the step time calculation:

T2=T−T1

Therefore, these durations T are active only when the motors (120) are moving in the same direction, since the pump's check valves (112 and 114) combine flows when the plungers (130) move in the same direction. It can be appreciated that the controller (150) stores pump specific constants that may include a system stiffness factor. The controller (150) also has pump specific characteristics including displacement per step and the number of steps per stroke as well as acceleration rates for the rate of change of step durations. The controller (150) also includes a desired output flow rate that may be constant or may be programmable and an output pressure that may be constant or programmable.

Examples of typical components for an exemplary metering pump include: a Hetai Stepper Motor model #57BYGH603; a McMaster-Carr Ball Screw model #; and a McMaster-Carr model #5966K16 Ball Nut.

Referring now to FIGS. 6 and 7, there are shown graphs for the output of the two pumping chambers and two plungers. FIG. 6 shows the outputs of each plunger and the combined output for a pump operating at high pressure.

Zone 1 is the beginning of Plunger 1 pressure stroke. During Zone 1 the plunger of Plunger 1 is moving forward and building pressure while the fluids compress and the chamber expands. During this period, there is no flow exiting the chamber, so the Plunger 2 continues at the maximum output. As soon as Plunger 1 pressure reaches the output pressure at the end of Zone 1, Plunger 2 slows its flowrate rapidly to offset the flow starting from Plunger 1. In Zone 2 Plunger 2 continues to lower its flow to zero while Plunger 1 increases at rates that result in a constant combined flow. The plunger velocities at the start of Zone 2 can be adjusted to match the change in flow when check valve the pumping chamber of Plunger 1 opens. When Plunger 2 stops at top dead center Plunger 1 is producing full flow. Plunger 1 continues output at a constant rate for the duration of Zone 3. During Zone 3 Plunger 2 makes its suction stroke, traveling to bottom dead center and then starts moving forward to build pressure by the end of Zone 3.

Referring now to FIG. 7, there are shown the outputs of each plunger and the combined output for a pump operating with low pressure where fluid compression is insignificant. It will be appreciated that such conditions are a special case of the graph in FIG. 6 where Zone 1 has a length of zero because there is no pressure build up. Zone 2 begins where Plunger 1 starts increasing plunger speed up to the maximum output. At the same time in Zone 2 Plunger 2 is decreasing so the combined output is constant. Zone 3 is where Plunger 1 is producing full flow and Plunger 2 is producing no flow while it is in its suction stroke.

The present invention has been described with stepper motors (120, 220), which deliver a fixed displacement per step and are very simple to control. However, stepper motors are relatively inefficient for certain applications. In small metering pumps this is not a big factor, but in larger pumps energy losses could make stepper motors impractical. In such applications a servo motor system could be used. A system using servo motors would include variable speed control of the motor and position encoders to communicate to the microcontroller how fast to run the motor in the various ramp zones.

It can be appreciated that a minimum of two pumping chambers and associated plungers are required to take advantage of the overlap and modified control to produce a steady output flow. This is also the simplest configuration and typically the least expensive. However, in order to have the pressure strokes overlap, the intake strokes of a two plunger pump would not overlap, which may result in a moment of zero flow. For most metering applications, this is negligible and not important. However, for some applications, this may be important and require a different approach. Applications that have a sensitive or viscous fluid may require a smooth inlet flow. For such applications, a three pumping chamber pump may be used in which three of the plungers such as shown in FIG. 1 or FIG. 4 are utilized to create a pulseless inlet flow as well as a pulseless discharge. Moreover, it can be appreciated that many pumps may include a larger number of pumping chambers and associated plunger including five pumping chambers and plungers to replace conventional five cylinder pumps. For a pump with three plungers, the pump stroke cycle can be timed with the overlap similar to a shaft driven pump where the strokes have a phase difference 120 degrees apart or ⅓ of a pump cycle apart. Therefore, there is overlap timing on both the pressure and suction strokes of each plunger. The speeds during the overlaps are calculated in the same manner for both the pressure and suction strokes.

As shown in FIGS. 8 and 9, the flow outputs for a three plunger pump overlap. As shown in FIG. 8, the horizontal axis of the graph represents 300 time units for one pump cycle. The vertical axis is output or plunge velocity. All positive velocities are pressure strokes and all negative velocities represent suction strokes. In FIG. 8, the pump is operating at a low pressure so there is no pressure building zone. The overlap zones for two plungers are spaced so that they occur during the time that the third plunger is operating alone on the opposite stroke. When Plunger 1 is providing full output, Plunger 2 and Plunger 3 are combining their intake strokes.

Referring now to FIG. 9, a pump cycle for a three plunger pump is shown operating at high pressure. It can be appreciated that Zones 1, 2 and 3 are the same as the Zones for the two plunger pump shown in FIG. 6 in which Zone 1 is the pressure build zone, Zone 2 is the combined flow zone and Zone 3 is the portion of the stroke in which one plunger is providing full flow. It can also be appreciated that FIG. 9 shows that shorter Zones 1 and 2 can be used followed and then a dwell period can be added following Zones 1 and 2 at the top and bottom of the strokes. This is shown at the zero velocity line in FIG. 9 where the plungers would be at top dead center or bottom dead center. In a manner similar to that shown in FIG. 6 for a two plunger pump, each of the constant speed zones are extended to cover the pressurization zone of the next plunger. This can also be seen on the suction side of the stroke where the constant negative velocity is extended on each plunger while the next plunger is lowering its pressure as it retracts from top dead center. It can be appreciated that the combined total flow therefore is substantially pulseless and provides improved control relative to what is possible with the prior art or any combination thereof.

In operation, the controller would include various constants and parameters related to the particular pump being controlled and the desired output flow and operating flow and pressure. The controller would calculate:

-   -   Step duration for output flow (Tf)     -   Number of steps for pressure build during Zone 1 (N11)         (decreasing step duration)     -   Time for zone 1 (Tz1) based on N11 and acceleration rate     -   Number of steps for the flow producing plunger during Zone 1         (N21) N21=Tz1/Tf (at constant step duration (Tf).     -   Number of steps for overlap Zone 2 (N2)     -   Time for Zone 2 (Tz2)     -   Number of steps for the flow producing plunger during Zone 3         (N13)=Nf−(N11−N2)

Once the operating parameters have been calculated, the controller is able to operate the pump for a desired flow rate and pressure. Referring to FIG. 10, the operation begins by starting the pump and moving both plungers to bottom dead center. The second plunger is moved at a Tf step duration for Ns−(N2+N12) steps. This represents the start of Zone 1. A stroke begins by moving the plunger 1 with decreasing step duration (T1) for N11 steps. Then, at each step of plunger 1, T2 is calculated using the formula T2=Tf−T1. The controller communicates to the stepper motor to continue moving plunger 2 with the increasing T2 step durations until top dead center at the end of Zone 2.

When plunger 1 reaches the end of Zone 2, the controller instructs the stepper motor to continue for N13 steps at Ts step duration to the end of Zone 3. Then, when plunger 2 reaches the end of Zone 2, plunger 2 is at top dead center. As the controller detects plunger 2 reaching top dead center, the controller instructs the stepper motor to reverse its direction to arrive at bottom dead center at a time to start its pressure stroke before plunger 1 finishes its pressure stroke. These strokes are repeated alternating plunger 1 and plunger 2. The resulting flow is substantially pulse free and achieves the desired flow rate and output pressure.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

What is claimed is:
 1. A positive displacement pump, comprising: a plurality of pumping chambers; a plunger associated with each pumping chamber, each of the plungers moving in a reciprocating motion; a variable speed motor driving each plunger, the variable speed motor varying speed during each stroke; and a controller controlling speed and direction of each variable speed motor.
 2. The pump according to claim 1, the variable speed motor being coupled to a leadscrew having a guide rod threadably mounted on the leadscrew to move along the leadscrew as the leadscrew rotates.
 3. The pump according to claim 2, wherein the guide rod is connected to the plunger.
 4. The pump according to claim 3, further comprising a position sensor in communication with the controller.
 5. The pump according to claim 1, wherein the variable speed motor comprises a stepper motor.
 6. The pump according to claim 5, wherein the stepper motors have variable rotation in one direction for a pressure stroke and in an opposite direction for the suction stroke and the controller is adapted to change speeds of each motor during the stroke so that the combined output of the plungers produce a constant flow.
 7. The pump according to claim 1, wherein the pump comprises two plungers, wherein a speed profile for a pressure stroke includes a pressure ramp portion at a beginning of the stroke and at an end of the stroke, wherein pressure builds up to a discharge pressure at a start of the stroke and decays at an end of the stroke, but no flow exits a first one of the plungers, and wherein the other one of the plungers produces full flow during periods of pressure ramping.
 8. The pump according to claim 1, further comprising a pressure input.
 9. The pump according to claim 1, further comprising a load input.
 10. The pump according to claim 1, further comprising a pressure input or a load input.
 11. The pump according to claim 1, further comprising a pressure input and a load input.
 12. The pump according to claim 1, wherein the controller is adapted to slow the motor to slow rotation speed of the leadscrew as the plunger nears top dead center and stops at top dead center first direction.
 13. The pump according to claim 1, wherein the controller is adapted to slow the motor to slow rotation speed of the leadscrew as the plunger nears bottom dead center and stops at bottom dead center second direction.
 14. The pump according to claim 1, wherein the pump comprises three plungers and wherein the guide rods have ⅓ of cycle different from each other.
 15. The pump according to claim 1, wherein the variable speed motor comprises a servo motor.
 16. A method for controlling a pump, the pump having a first pumping chambers and an associated first plunger, and a second pumping chamber and an associated second plunger, each plunger being driven by an associated variable speed motor, and a controller; the method comprising: controllably driving a first variable speed motor to vary displacement and speed of the associated first plunger; controllably driving a second variable speed motor to vary displacement and speed of the associated second plunger; the speed of displacement of each variable speed motor being varied as the plungers approach top dead center and bottom dead center and coordinated so that pump outflow is substantially constant.
 17. The method according to claim 16, wherein determines flow by a controller with the formula: dV=P*V/K where: K and V are constants stored in the microcontroller for the pump. P=the output pressure and can be entered by the operator or input from a transducer. K=the combined bulk modulus of the system which includes the fluids and components that deflect during the pressure stroke. V=pumping chamber volume dV=the change in volume due to compression of the fluids and expansion of the pumping chamber.
 18. The method according to claim 16, wherein the variable speed motors comprise stepper motors. 